Transcript Document

Condensations with secondary amines give enamines.
Aldehydes and ketones react with secondary amines to produce enamines. In this
reaction, water is eliminated between the hydroxyl group and the neighboring
carbon atom, rather than the nitrogen atom.
Enamine formation is reversible and hydrolysis occurs readily in the presence of
aqueous acid.
17-10 Deoxygenation of the Carbonyl Group
Strong base converts simple hydrazones into hydrocarbons.
Hydrazones are formed from the condensation of an aldehyde or ketone with
hydrazine.
When treated with a base at elevated temperatures, hydrazones decompose,
liberating the corresponding hydrocarbon and nitrogen gas. This reaction is known
as the Wolff-Kishner reduction.
Wolff-Kishner reduction aids in alkylbenzene synthesis.
The Wolff-Kishner deoxygenation is frequently used in lieu of Friedel-Crafts
alkanoylation, particularly for acid-sensitive, base-stable substrates.
17-11 Addition of Hydrogen Cyanide to Give
Cyanohydrins
Hydrogen cyanide adds to the carbonyl group to form cyanohydrins.
Slow addition of HCl to an excess of NaCN is typically used to generate HCN in a
moderately alkaline mixture.
This reaction requires the presence of both a free cyanide ion and undissociated
HCN (satisfied by maintaining a moderately basic pH).
Cyanohydrins are useful intermediates because the nitrile group can be modified
by further reaction.
17-12 Addition of Phosphorous Ylides: The Wittig
Reaction
A phosphorus ylide contains a carbanion that is stabilized by an adjacent,
positively charged phosphorous group.
A ylide is capable of attacking aldehydes and ketones in a reaction called the
Wittig reaction, which allows the synthesis of alkenes from aldehydes and
ketones.
Deprotonation of phosphonium salts gives phosphorous ylides.
Ylides can be prepared from haloalkanes by a two-step sequence:
The phosphonium salt can be deprotonated by bases such as alkoxides, sodium hydride or
butyl lithium, giving the ylide:
The Wittig reaction forms carbon-carbon double bonds.
In the Wittig reaction, an ylide reacts with an aldehyde or ketone by coupling the
ylide carbon to that of the aldehyde or ketone:
In the Wittig reaction, the position of the newly formed double bond is
unambiguous:
The mechanism of the Wittig reaction involves the formation of a phosphorous
betaine, a dipolar species of a type known as a zwitterion.
Wittig reactions can be carried out in the presence of ether, ester, halogen, alkene
and alkyne functions.
Wittig reactions are only sometimes stereoselective; cis-trans mixtures of alkenes
may be produced.
17-13 Oxidation by Peroxycarboxylic Acids: The
Baeyer-Villiger Oxidation
The Baeyer-Villiger oxidation converts a carbonyl function to an ester using a
peroxycarboxylic acid.
The mechanism of this oxidation involves a cyclic transition state in which an alkyl
group shifts from the original carbonyl carbon to oxygen to give an ester.
Cyclic ketones are converted into cyclic esters. Attack is at the carbonyl rather
than a carbon-carbon double bond, if present.
Unsymmetric ketones can, in principle, lead to two different esters, but only one
is observed:
Some substituents migrate more easily than others. The migratory aptitude of
various groups suggests that the migrating carbon possesses carbocationic
character in the transition state.
17-14 Oxidative Chemical Tests for Aldehydes
Two simple chemical tests can indicate the presence of the aldehyde function. In
both tests, the aldehyde is oxidized to the carboxylic acid.